I did not find your man O.M. Watkins in Crute's "Confederate Staff Officers", or in "List of Field Officers, Regiments & Battalions in the Confederate States Army" from J.M. Carroll & Company, or in "Price's Lieutenants" by Peterson, McGhee, et al.

However, he is listed in the "Index to the Offical Records" in several volumes, including the following entry from Vol XV, p. 1074:

HEADQUARTERS EASTERN SUB-DiSTRICT OF TEXAS, Houston, May 4, 1863. Capt. EDMUND P. TURNER, Assistant Adjutant- General: CAPTAIN: Under the orders of Lieutenant-General Smith, commanding department, &c., I beg to say to the major-general commanding that I am sending to Niblett’s Bluff on the Sabine River, all the disposable force at my command, including Major Wilson’s, battery. I beg to know of the major-general commanding as to the propriety of placing Maj. 0. M. Watkins, who wishes to go, in command of that battery. I have endeavored to keep the major-general commanding advised of every important matter that has occurred. I have the honor to be, very respectfully, your obedient servant, W. R SCURRY, Brigadier-General, Commanding, &c